Ball Valve Seat Selection: PTFE versus Metal Seats

ted wang
Jun 5
2 min read

Seat selection in ball valves is one of the most consequential engineering decisions affecting valve performance, maintenance, and lifecycle cost. The choice between PTFE (soft) seats and metal seats should be based on service conditions rather than cost alone.

PTFE Seat Characteristics

Excellent chemical resistance to most process fluids
Low coefficient of friction enabling easy operation
Achieves bubble-tight ASME FCI 70-2 Class VI shutoff
Temperature limit: approximately -46 to 200 deg C for standard PTFE
Not suitable for abrasive media that scratches the seating surface
Glass-filled or carbon-filled PTFE improves creep resistance

Metal Seat Characteristics

Required for temperatures above 200 deg C
Resistant to abrasive and erosive media
Fire-safe: maintains shutoff after fire destroys soft seat
Typically achieves Class IV to V leakage (not bubble-tight)
Requires lapping and careful surface finish to achieve sealing
Stellite, Inconel, or hardened stainless steel are common seat materials

Service Condition Guidance

PTFE seats are appropriate for clean to moderately contaminated services at temperatures below 200 deg C where bubble-tight shutoff is required. Metal seats should be specified for high-temperature service, fire-safe requirements, and media containing abrasive particles.

Fireproof and Fire-Safe Requirements

API 607 defines fire-testing requirements for quarter-turn valves. A fire-safe ball valve typically has a primary PTFE seat for normal operation and a secondary metal-to-metal seal that engages in the event of fire. After fire testing, leakage must not exceed specified limits per API 607.

Trunnion vs Floating Ball

Floating ball designs press the ball against the downstream seat under pressure, providing self-energized seating. Trunnion-mounted balls are supported by stem trunnions and rely on spring-loaded seats to maintain sealing. Floating ball designs can achieve better low-pressure sealing, while trunnion designs handle higher pressure more effectively.

Maintenance

Soft seat replacement is a common maintenance task for ball valves. Seat rings can typically be replaced without removing the valve from the line in split-body or top-entry designs. Metal seats that are scratched or damaged typically require re-lapping or replacement.